E.R.
I would ask how they got the idea for that kind of play. I would not imply that there is anything wrong or shocking about what they are doing... but I would ask where they got the idea, and why they wanted to do that.
I think to some extent, curiosity like that is totally normal. But they really should not know about being on top of each other or kissing at that age... so somewhere along the line, they saw something or heard something. I would try in a very nonchalant way to find out where. Then you can explain that ADULTS who are in love do these things with each other, and it's how they show each other that they love each other. And ask him if he has any questions. By avoiding the subject or talking around it, you are only leaving room for curiosity and experimentation. So as hard as it is, you have to address it head on.
There are books out there, I am sure, for this kind of discussion. Maybe it would help if Dad talked to him about it. I am sure he gets erections and is at an age where he is wondering about why??? So Dad can explain all of that stuff. But, if Dad isn't willing... then it falls on you, mom. I know it's awkward, but you need to talk about it and take the mystery out of it so he doesn't feel the need to experiment and find out on his own.